Activation Keys

using System;
using System.Text.RegularExpressions;
using System.Linq;
using System.Collections.Generic;

namespace _02._Activation_Keys
{
    class Program
    {
        static void Main()
        {
            var input = Console.ReadLine().Split("&");
            var result = new List<string>();

            for (int i = 0; i < input.Length; i++)
            {
                var firstPattern = @"^[a-zA-Z0-9]{16}$";
                var secondPattern = @"^[a-zA-Z0-9]{25}$";

                if (Regex.IsMatch(input[i], firstPattern) || Regex.IsMatch(input[i], secondPattern))
                {
                    var word = input[i].ToUpper().ToList();
                    for (int j = 0; j < word.Count; j++)
                    {
                        if (char.IsDigit(word[j]))
                        {
                            var symbol = (9 - int.Parse(word[j].ToString())).ToString();
                            var symbolInChar = Convert.ToChar(symbol);
                            word[j] = symbolInChar;
                        }
                    }
                    if (word.Count == 16)
                    {
                        word.Insert(4, '-');
                        word.Insert(9, '-');
                        word.Insert(14, '-');
                    }
                    else if (word.Count == 25)
                    {
                        word.Insert(5, '-');
                        word.Insert(11, '-');
                        word.Insert(17, '-');
                        word.Insert(23, '-');
                    }

                    var wordInString = new string(word.ToArray());
                    result.Add(wordInString);
                }
            }

            Console.WriteLine(string.Join(", ", result));
        }
    }
}